Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Your mothers sister is your Aunt
Your sister-in-law's son is your nephew.
Your husband's sister's son is your nephew, just as your sister's son is your nephew.
The sister's son would be the nephew of her brother.
Aunt Aunty Auntie
If your sister-in-law is the sister of your spouse, her son is not related to your sister's son. If your sister-in-law is the wife of your brother, her son and your sister's son are first cousins.
Your nephew.
Your sister's son is your nephew. You are his aunt if you are a woman, or his uncle if you are a man.
Your sister's son is your nephew. You are his aunt, if you are female, or his uncle, if you are male.
Our cousins are our mothers sisters son :)
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Your sister's son and your daughter are first cousins to each other.